Reduction-diffusion process (RD process) is known for producing Sm—Co based magnet alloys, and is now in practical use. The RD process involves heating oxides of the rare earths and the other constituent metals with a reducing agent such as metallic calcium or calcium hydride, in an inert atmosphere to reduce the rare earth oxides into metals, which are simultaneously diffused into the other constituent metals. The reaction product is then cooled to the room temperature, and introduced into water to dissolve and remove the reduction products, such as CaO, CaO.2CaCl2, and Ca(OH)2, and unreacted residual metallic calcium. The resulting product may optionally be subjected to an acid treatment.
Another method in practical use is a casting method which involves blending the constituent rare earth metals and other metals, or master alloys consisting of these constituent metals, at a given composition, high-frequency induction melting, and casting the alloy melt in a mold to have a thickness of about 50-100 mm. Also proposed is a method, which has been under consideration, and involves blending the constituent rare earth metals and other metals, or master alloys consisting of these constituent metals, at a given composition, and high frequency induction melting, as in the above mold casting method, and then rapidly cooling and solidifying the alloy melt continuously by strip casting on a single or double rolls or on a disk.
In the Sm—Co based magnet alloys produced by the RD process, the adhesive and aggregates, such as CaO, CaO.2CaCl2, and Ca(OH)2 or unreacted residual metallic calcium, have not been removed completely, and are present at about 50 to 2000 ppm, which lower corrosion resistance. Further, since the RD process involves contacting the alloy with water or the like medium, the oxide content of the resulting Sm—Co based magnet alloy becomes as high as 1000 to 2500 ppm. This increases the volume of the non-magnetic phase, and thus deteriorates magnetic properties.
The mold casting, on the other hand, solves the problem of lowered corrosion resistance encountered in the RD process. However, when a cast ingot is pulverized, coarse particles are in the pulverized particles. These coarse particles lower magnetic properties to a similar level as can be achieved by the RD process.
A Sm—Co based magnet alloy prepared by conventional strip casting contains a relatively large amount of R2M7 and RM3 phases having lower magnetic moment than that of RM5 phase. Thus magnetic properties of this alloy are inferior to those produced by the RD process or mold casing.